Wafer cleaning apparatus

ABSTRACT

A wafer cleaning apparatus includes a wafer supporting device for supporting a wafer to be cleaned, and sponges for cleaning the wafer. The sponges are arranged in contact with either surface of the wafer and are rotated along rotational axes that are parallel to the wafer. A sponge positioning device pushes the sponges against the wafer. A wafer rotation drive is adapted to rotate the wafer at a constant speed, and a sponge rotation drive system is adapted to rotate the sponges at a constant speed. A closed loop controller receives motor current signals from the wafer rotation drive and/or the sponge rotation drive system, and the closed loop controller provides an adjustment signal to the sponge positioning device. The adjustment signal is for adjusting the friction between the sponges and the wafer

BACKGROUND OF THE INVENTION

[0001] Field of the Invention

[0002] The present invention relates to a wafer cleaning apparatus whichcan be suitably used for post CMP (Chemical Mechanical Polishing)cleaning.

[0003] Generally, after Chemical Mechanical Polishing of a wafer, a veryefficient removal of particles from the wafer surface is required.

[0004] To this end, as is shown in FIG. 2, the wafer 3 is placed in acleaner box in a vertical orientation and two cylindrical PVA (polyvinylacrylate) sponges 1, 2 are brought into physical contact with eithersurface of the wafer 3. The cylinder axes of the sponges 1, 2 areparallel to the wafer 3, and the sponges 1, 2 are rotating in oppositedirections. Accordingly, the wafer 3 is standing between rotatingsponges 1, 2 pushing toward the wafer surface. Moreover, deionized waterand/or chemicals are applied to the wafer 3 as well as to the sponges 1,2 in order to remove particles from the wafer 3 and the sponges 1, 2.

[0005] In order to cover the whole wafer surface, the wafer 3 also hasto be rotating during the cleaning process. For this purpose, the wafer3 is supported by rollers 4 which make the wafer 3 rotate. The rollers 4as well as the rotational axes of the sponges 1, 2 are driven by servomotors.

[0006] In order to obtain satisfactory cleaning results, constantpressure and friction conditions between the sponges 1, 2 and the wafer3 regardless of changes of the mechanical characteristics of the spongesare required.

[0007] To satisfy this requirement, the pressure being applied in orderto push the sponges 1, 2 towards the wafer 3 may be controlled by apressure regulator.

[0008] Alternatively, the pressure conditions are determined bypresetting a gap between both sponges 1, 2. To this end, the sponges 1,2 are moved towards a hard stop system at a maximum pressure. The fixeddimensions cause an overlap between the outside circumference of thesponge 1 or 2 and the wafer 3. Thus, a certain pressure between sponges1, 2 and the wafer 3 is generated.

[0009] However, the presetting of the gap is a very complicated processsince the hard stop can only manually be adjusted. In particular, thewidth of the gap has to be adjusted at a very high precision and thewafer 3 must accurately be centered between the two sponges 1, 2.

[0010] In summary, the known methods of applying a constant pressurebetween the sponges and the wafer involve the following drawbacks:

[0011] There is no control of the real force and friction conditionsbetween the sponges and the wafer. Usually, changes in the mechanicalcharacteristics of the sponges or the removal of the top layer of thewafer lead to differences in the friction conditions between the spongesand the wafer. Hence, different cleaning results are achieved if thereis not any control of the mechanical parameters of the cleaning process.

[0012] There is no possibility of adapting the real friction conditionsto a changed process requirement. For instance, at some point of thecleaning process, special friction conditions might become useful forobtaining an optimum cleaning result.

[0013] In particular, when using the hardstop system, the cleaningconditions are preset as soon as new sponges are installed. However, inthe course of time, the compression may reduce the outer diameter of thesponges. These changes will directly impact the pressure conditions, andthus, the cleaning results.

[0014] Accordingly, a lot of maintenance activities are required. Inparticular, the accuracy of the gap between the sponges must befrequently checked and reset.

[0015] Nevertheless, there is no control of the cleaning parameters overthe lifetime of the sponges.

SUMMARY OF THE INVENTION

[0016] It is accordingly an object of the invention to provide a wafercleaning apparatus which overcomes the above-mentioned disadvantageousof the prior art apparatus of this general type. In particular, it is anobject of the invention to provide a wafer cleaning apparatus having aclosed loop control for controlling the friction existing between waferand sponges.

[0017] With the foregoing and other objects in view there is provided,in accordance with the invention, a wafer cleaning apparatus forcleaning a wafer having two surfaces. The wafer cleaning device includesa wafer supporting device for supporting the wafer to be cleaned, andsponges for cleaning the wafer. Each of the sponges is configured incontact with either one of the surfaces of the wafer. The sponges arerotatably mounted with rotational axes that are parallel to the wafer.The wafer cleaning device also includes a sponge positioning device forpushing the sponges against the wafer; a wafer rotation drive adapted torotate the wafer at a constant speed; a sponge rotation drive systemadapted to rotate the sponges at a constant speed; and a closed loopcontroller receiving motor current signals from a rotation driveselected from the group consisting of the wafer rotation drive and thesponge rotation drive system. The closed loop controller provides anadjustment signal to the sponge positioning device. The adjustmentsignal is for adjusting friction between the sponges and the wafer. Thesponges are preferably cylindrical and the motor current signalsrepresent friction data between the wafer and the sponges.

[0018] The motor current signals from the wafer rotation drive and/orsponge drive are used to online control the frictional conditionsbetween the wafer and the sponges. These signals, which represent thetorque necessary to maintain a constant driving speed, and thus,represent the real frictional conditions between the wafer and thesponges, are given to a closed loop control to actively and onlinecorrect changes of the cleaning conditions. Thus, changes in themechanical characteristics of the sponges do not impact the cleaningprocess, and the frictional conditions can always be kept constant overthe lifetime of the sponges.

[0019] The present invention can be implemented using differentmechanisms for pushing the sponges against the wafer surfaces. Forinstance, the adjustment signal provided by the closed loop control canbe given to a pressure regulator which sets the pressure applied to thesponges on the basis of the adjustment signal.

[0020] Alternatively, the adjustment signal provided by the closed loopcontrol can be given to a positioning system that, preferablyelectrically, controls the position of the sponges.

[0021] In particular, this electric positioning system can include ahard stop system for setting the gap between the two sponges. In thiscase, the hard stop is specially designed and a motor drive foradjusting the position of the hard stop is provided. The position of thehard stop will determine the gap between the two sponges as will beexplained later in more detail.

[0022] However, any arbitrary system for positioning the sponges inrelation to the wafer can be used as well.

[0023] In summary, the present invention provides the followingadvantages:

[0024] It is possible to get information about the real frictionalconditions between the sponges and the wafer during the wafer cleaningprocess. Thus, an online feedback signal about a very important cleaningparameter can be received and this signal can be used for closed loopcontrol of the mechanical cleaning parameter.

[0025] Consequently, taking into account the closed loop control of therotational speed of the sponges, which has normally been used in thestate of the art, the mechanical part of the cleaning process iscompletely controlled.

[0026] The present invention enables an online correction, and thus, animmediate reaction if the friction conditions are changing.

[0027] Thus, the same frictional conditions can always be maintainedwhereby the stability of the process can be remarkably improved.

[0028] Moreover, an automatic adjustment of the process parameterrepresenting the friction becomes possible. Thus, an adaptation tovarying process requirements becomes possible, for example, withdifferent products, different frictional conditions can be adjusted.

[0029] The present invention enables an online control of the spongebehavior over the whole lifetime of the sponge.

[0030] In addition, the lifetime of the sponges can be increased byapplying different frictional conditions as soon as the sponges havereached their standard lifetime.

[0031] Moreover, according to a preferred embodiment of the presentinvention, the desired frictional conditions can be preset in a recipeor instruction set and can thus be adapted to different cleaningrequirements.

[0032] If additionally the positioning device includes a hard stopsystem, the following additional advantages are achieved:

[0033] A closed loop pressure control system is not necessary.

[0034] Moreover, the hard stop position can be additionally monitored.

[0035] In addition, the step of presetting the gap, which is acomplicated process, can be avoided.

[0036] Hence, the maintenance activities can considerably be reduced. Inparticular, frequent checks of the adjustment of the gap between thesponges are no longer necessary.

[0037] In summary, the implementation of the present invention willincrease the yield, improve the wafer cycle time, and consequently,reduce the cost.

[0038] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0039] Although the invention is illustrated and described herein asembodied in Wafer Cleaning Apparatus, it is nevertheless not intended tobe limited to the details shown, since various modifications andstructural changes may be made therein without departing from the spiritof the invention and within the scope and range of equivalents of theclaims.

[0040] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 is a block diagram illustrating closed loop control of awafer cleaning apparatus;

[0042]FIG. 2 illustrates the post CMP cleaning process to which thepresent invention usually is applied;

[0043]FIG. 3 illustrates a possible implementation of the hardstopsystem used in the wafer cleaning apparatus; and

[0044]FIG. 4 illustrates a further possible implementation of thehardstop system used in the wafer cleaning apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is shown a software controller 12which receives a current signal from the wafer rotation drive 10 and/orthe sponge rotation drive system 11. The current signal supplied by thewafer rotation drive 10 or the sponge rotation drive system 11represents the torque necessary to maintain the wafer rotation or thesponge rotation at a constant speed, and thus, directly represents thereal frictional conditions between the sponge and the wafer. Usually,the wafer and the sponges are driven by servo drives which provideanalog output signals that are proportional to the motor current.

[0046] In the described embodiment of the present invention, the numberof sponges used is two, and the sponge rotation drive system 11 includesone single servo drive for rotating both sponges. Accordingly, since thefrictional conditions of the two wafer surfaces, to which the spongesare pushed, normally differ, an average value of the real frictionalconditions of the two wafer surfaces is measured.

[0047] However, any arbitrary even number of sponges can be used, andthe sponge rotation drive system 11 can include more than one servodrive, for example, one servo drive for the sponges on each wafersurface.

[0048] The software controller 12 compares the friction data representedby these current signals with preset friction conditions. Among others,these preset friction conditions take into account that the measuredfriction data are an average value as mentioned above.

[0049] According to a preferred embodiment of the present invention, arecipe or an instruction set representing the friction conditions whichare adapted to different cleaning requirements, in particular, fordifferent layers, can be stored in a storage means which is included inthe software controller 12.

[0050] The software controller provides a correction signal to apositioning device 13 for pushing the sponges against the wafer. Thepositioning device 13 controls the pressure between the sponges and thewafer on the basis of this correction signal so that finally, a presetvalue of friction between the sponges and the wafer is achieved. Thiscan be accomplished by either adjusting the position of the sponges orby adjusting the pressure at which the sponges are pushed against thewafer surface.

[0051] In more detail, if the motor current is lower than the presetvalue, the sponges will be moved further toward the wafer or thepressure of the sponges will be enhanced. On the other hand, if themotor current is higher than the preset value, the sponges will be movedaway from the wafer or the pressure of the sponges will be reduced.

[0052]FIGS. 3 and 4 illustrate embodiments of the present invention inwhich the positioning device is implemented as a device for setting theposition of the sponges that includes a hardstop 5 and a motor drive 6.

[0053] In these embodiments, as is shown in FIGS. 3 and 4, the twosponges 1, 2 are held by shafts 7, 8. When the cleaning process isstarted, the shaft system keeping the sponges 1, 2 in position is movedpneumatically towards the hardstops 5 at a maximum pressure.Accordingly, the sponges 1, 2 are pushed towards the wafer 3 and thepressure between the sponges 1, 2 and the wafer 3 is set by setting thegap which is determined by the hardstop position.

[0054] If this hard stop system is used the pressure applied between thewafer 3 and the sponges 1, 2 can easily be adjusted and no complicatedclosed loop pressure control system is necessary.

[0055] As is illustrated in FIG. 3, the hardstop 5 can be implemented asan elliptical cam plate, and the two shafts 7, 8 are arranged inparallel. In this case, the rotational angle of the cam plate and, thus,the distance between the two shafts 7, 8 and, consequently, the gapbetween the two sponges 1, 2 is adjusted by the motor drive 6. Hence,the minimum distance between the two shafts 7, 8 is equal to the minoraxis of the ellipse, and the maximum distance between the two shafts 7,8 is equal to the major axis of the ellipse.

[0056] Alternatively, as is depicted in FIG. 4, the hardstop 5 can beimplemented as a truncated cone, with the two shafts 7, 8 arranged in acrossing manner. Each of the shafts 7, 8 is in contact with the envelopeof the cone. In this case, the position of the hardstop 5, and thus, thecrossing angle of the two shafts 7, 8, and consequently, the gap betweenthe two sponges 1, 2 is adjusted by a spindle 9 which is driven by themotor drive 6.

[0057] According to the present invention, as an alternative to the hardstop system, the positioning system can also be implemented as a set ofspindles, each physically connecting the sponges and a set ofcantilevers for supporting the sponges. The cantilevers are held inparallel at a constant distance. By adjusting these spindles, theposition of the sponges can appropriately be set. This implementationinvolves the advantage that each of the sponges can separately beadjusted. Moreover, in contrast to the implementation using the hardstop system, it is not necessary to center the wafer between the twosponges.

We claim:
 1. A wafer cleaning apparatus for cleaning a wafer having twosurfaces, comprising: a wafer supporting device for supporting the waferto be cleaned; sponges for cleaning the wafer, each of said spongesconfigured in contact with either one of the surfaces of the wafer, saidsponges rotatably mounted with rotational axes that are parallel to thewafer; a sponge positioning device for pushing said sponges against thewafer; a wafer rotation drive adapted to rotate the wafer at a constantspeed; a sponge rotation drive system adapted to rotate said sponges ata constant speed; and a closed loop controller receiving motor currentsignals from a rotation drive selected from the group consisting of saidwafer rotation drive and said sponge rotation drive system; said closedloop controller providing an adjustment signal to said spongepositioning device, said adjustment signal for adjusting frictionbetween said sponges and the wafer.
 2. The wafer cleaning apparatusaccording to claim 1, wherein said sponge positioning device includes adevice for setting a position of said sponges.
 3. The wafer cleaningapparatus according to claim 2, wherein said device for setting theposition of said sponges includes a hardstop and a motor drive.
 4. Thewafer cleaning apparatus according to claim 3, wherein said hardstop isan elliptical cam plate, said elliptical cam plate mounted at arotational angle that is adjusted by said motor drive.
 5. The wafercleaning apparatus according to claim 3, comprising: two crossing shaftsholding said sponges; and a spindle that sets a position of saidhardstop; said hardstop is formed as a truncated cone.
 6. The wafercleaning apparatus according to claim 1, wherein said sponge positioningdevice includes a pressure regulator for controlling a pressure at whichsaid sponges are pushed against the wafer.
 7. The wafer cleaningapparatus according to claim 1, wherein said closed loop controllerincludes storage means for storing a recipe representing frictionalconditions that are adapted to different cleaning requirements.
 8. Thewafer cleaning apparatus according to claim 1, wherein said closed loopcontroller includes storage means for storing instructions representingfrictional conditions that are adapted to different cleaningrequirements.
 9. The wafer cleaning apparatus according to claim 1,wherein said closed loop controller includes a storage device forstoring instructions representing frictional conditions that are adaptedto different cleaning requirements.